I can't quite give you the perspective of someone who in in one of the frats, but I can give you the perspective of someone who is not in one.
I'm currently between my first and second year at USC and I decided not to join a frat. I'm pretty sure Psi Omega and Delta Sigma Delta both charged the same price of $250 for a lifetime membership (I could be off by like $25 either way, but I'm pretty sure it was $250). For that price you get to go there pretty much whenever you want, they have parties usually once a week I think, and you get use of the lab. Both of those frats also host parties throughout the year for people that aren't in the frat. They also both typically host tailgates before the football games for everyone.
The SIM lab at the school is open until midnight and I've yet to stay that late (who knows what will happen with posterior fixed). Also, the other "benefit" of the frats are the study packets that they send out to their members. I have friends in all the frats thus I have get all the packets anyway.
Ultimately I don't think I've missed much by not joining the frats, however I'm obviously biased as I have not been involved with some of the internal frat activities. I'm also not hugely into the frat scene anyway, so depending on your personality your mileage may vary.
The third frat, Alpha Omega, isn't really a typical fraternity; it's more like an organization. I don't know if there are membership dues for AO, but I do know that they don't have a house or lab so it's basically the fellowship and study packets.
Ok, so you said you want to study and also do social events, which frat is most like that? Well, I'd probably say DSD is more of the "studious" frat and PsiO is more of the "party" frat. With that being said, they're both frats made up of people that worked really hard to get into a dental school that's hard to get into, it's not like they're getting wasted each and every night.
Hope that helps.
